1. Field of the Invention
This invention relates to a color filter substrate for a liquid crystal display device, and to a liquid crystal display device which is provided with this color filter substrate. In particular, this invention relates to a color filter substrate for a liquid crystal display device which is excellent not only in contrast but also in oblique visibility and to a liquid crystal display device having such features.
2. Description of the Related Art
In recent years, flat-panel display devices such as a liquid crystal display device are increasingly demanded to enhance the picture image and power-saving thereof and to lower the manufacturing cost thereof. In the case of the color filter, it is demanded to exhibit sufficient color purity, sufficient brightness and high contrast.
Especially, in the case of a large television or a high-image-quality monitor, where the display contrast is not less than 2000, there is now a demand to exhibit not only a high front face contrast but also a very high level of display quality with respect to viewing angle characteristics including oblique viewing direction. In order to improve the contrast and visibility to attain such a high level, it is required to improve not only so-called front contrast, i.e. a difference between the white display and the black display when the liquid crystal display device is viewed from the front face thereof but also the oblique visibility so as to minimize the changes in colors and coloring of the black display when the liquid crystal display device is viewed from an oblique direction.
Depending on the kinds of color filter application, such as a television, a monitor and a mobile display device, the colors (red, green, blue, etc.) of the color filter are frequently required to be delicately adjusted. In spite of such a delicate adjustment of colors, it is still demanded to realize a wider viewing angle and a black display of higher quality.
With respect to the method of improving the oblique visibility, a technique is disclosed, wherein the difference is provided in retardations of the color layers such as red, green and blue pixels of a color filter to thereby improve the oblique visibility (see for example, JP-A 5-196930). There has been also tried to reduce the retardation that the color filter may exhibit, wherein a polymer having a planar structural group on its side chain is introduced into a color layer, or a birefringence-reducing particles having a birefringence which is opposite in sign to that of the polymer is introduced into the color layer (see for example, JP-A 2000-136253 and JP-A 2000-187114).
Further, there has been proposed an idea to incorporate a retardation-adjusting agent in the color layers of color filter, thus enabling each of color pixels to have a different retardation, thereby making it possible to improve the viewing angle compensation (oblique visibility) of black state of a liquid crystal display device without necessitating the provision of a polymeric liquid crystal layer in addition to the color layers or without necessitating the change of thickness in each of subpixels (see for example, JP-A 2008-20905, JP-A 2008-40486 and JP-A 2008-145868).
The conventional techniques described above however are accompanied with a problem that when it is tried to control the retardation of display pixels, various characteristics including the physical properties of color filter are caused to change. The reason is that when a side chain having a planar structural group is introduced into a polymer taking the role of pigment carrier in a coloring macromolecular thin film, the density, mechanical strength and chemical resistance of the thin film may be caused to deteriorate or the exposure sensitivity and developing properties of the thin film may be caused to change when creating a pattern by means of photolithography, thereby raising various problems in the manufacture of the color filter.
For example, the retardation-adjusting agent disclosed in JP-A 2008-145868 has a light absorption in a wavelength region of 250-400 nm, it is necessary to apply an excessive exposure and, hence, the productivity of color filter would be badly affected. Further, the additional incorporation of the retardation-adjusting agent or the birefringence-reducing particles, both being incapable of contributing in anyway to the development of the strength of film, in the color layer would deteriorate the mechanical strength, chemical resistance, adhesion of the color layer. Meanwhile, in the case of a liquid crystal display device designed to exhibit high image qualities, the display device is now demanded to exhibit not only a black display of high purity and high concentration but also very high contrast.
The present inventors have already found out that, in order to realize not only a black display having a very high contrast and pure black but also high quality oblique visibility, it is required to decrease the value of retardation in thickness direction Rth to nearly zero in all of every color pixels of color filter. More specifically, it is required to regulate the value of retardation in thickness direction Rth of every color pixels to the range of 0-3 nm. However, since the conventional red pigments are formed to have a relatively large birefringence Δn, it has been difficult to decrease the value of Rth to nearly zero. Further, even in the green layer, since green pigments or yellow pigments to be contained therein inherently have a large value as an absolute value of birefringence, it has been difficult, as in the case of the red layer, to decrease the Rth to nearly zero.
Furthermore, it has been difficult to obtain a color filter which is not more than 3 nm in the absolute value of retardation in thickness direction and which is capable of securing the photolithographic suitability thereof and overcoming the problem of oblique visibility while retaining a color filter contrast of as high as not less than 5000, furthermore not less than 9000.
In the case of a transparent resin film having a different birefringence or a different retardation in thickness direction Rth, it is generally possible to utilize so-called “additivity rule”, which make it possible to adjust the Rth through the lamination of the films. Even in the case of the color filter for a liquid display device, it is possible to confirm the usefulness of almost the same additivity rule as in the transparent resin film through the lamination of color layers each having a different retardation in thickness direction Rth. In the case of a pigment dispersion consisting of a dispersion of plural kinds of pigments differing in color however, no one has succeeded in confirming complete additivity rule as yet even though it is more likely possible to confirm the additivity rule as far as the mixing of pigments is concerned. Namely, the adjustment of the Rth has been performed empirically in most cases.
Further, even in the case where pigments are the same in color having the same pigment number, the retardation in thickness direction Rth may differ depending on the manufacturing method thereof and on the manufacturer thereof, raising confusion when manufacturing a photosensitive color composition.